1. Field of the Invention
The present invention relates to a power amplifier and to a nuclear spin tomography apparatus employing same. The power amplifier can be used in all applications in which high output voltages and currents must be provided, in particular for inductive loads. For example, the amplifier is suited for driving motors and actuators in automation engineering, traffic engineering and plant engineering; however, an application of the amplifier in medical technology as a gradient amplifier in nuclear spin tomography (magnetic resonance imaging) is provided in particular.
2. Description of the Prior Art
A nuclear spin tomography apparatus typically has an orthogonal gradient coil system that surrounds the patient chamber. For each gradient coil, a gradient amplifier is provided, via which the coil is supplied with a precisely regulated current. The precision and dynamic performance of the gradient current are decisive for the image quality. Thus, for example, currents up to 300 A must be maintained with a precision in the mA range, and for the production of sufficiently steep current edges, it must be possible for example to apply voltages up to over 1 kV to the gradient coil. In addition, the output current can have only a small residual ripple.
German OS 40 24 160 discloses a gradient amplifier that has a switched output stage in a bridge circuit with four FET power transistors and four unbiased diodes respectively in parallel thereto. For each direction of the load current, two transistors diagonally opposite one another in the bridge circuit are clocked periodically, and in addition two transistors connected in series in the bridge circuit are driven with opposite phase.
German OS 40 07 566, corresponding to U.S. Pat. No. 5,113,145, discloses a further gradient amplifier in which chokes are inserted between the bridge branches in order to avoid cross-currents. This gradient amplifier further has a specific construction for the reduction of parasitic inductances.
Due to the exacting requirements described above, however, these known gradient amplifiers are very complex and expensive. High costs are caused in particular by the expensive components required for the amplifier, the complicated driving, the considerable structural size and the cooling requirements.